1. Field of the Invention
The present invention relates to prosthetic devices and more particularly relates to an improved joint prosthesis that has improved shock absorption.
Even more particularly, the present invention relates to an improved joint prosthesis that includes articulating joint members, one of which flexes during impact to absorb shock, and wherein a gap between one joint component and another joint component allows deflection of one component into the gap area, to lessen impact stresses.
2. General Background
A tibial component for use in a knee prosthesis usually comprised of two parts, including a base plate which may be plastic or metal but which is usually metal, and a plastic plate which fits into or onto the base plate. The base plate carries some means for securing the base plate component to the bone, usually in the form of one or more screws or projections. Some base plates carry a pattern of fins which increases anchor strength and prevents rotation of the tibial component. The plastic plate can be prepared from high density polyethylene, and includes an upper articulating surface which is contoured to provide a surface on which the femur or femoral component can move.
Some patents (see eg. U.S. Pat. Nos. 4,865,407, 4,769,040 and 4,673,407 and European Patent No. 46926) discuss generally the problem of shock absorbing prostheses. U.S. Pat. Nos. 4,865,607 and 4,769,040 rely on a flexible metal or a reinforced resin layer to provide stress dissipating structure. U.S. Pat. No. 4,673,407 incorporates a spring to maintain a freely movable tibial prosthesis in position against the bone.
U.S. Pat. No. 4,865,607 describes a tibial plate which includes a reinforcement member capable of distributing the joint forces uniformally over the spongiosa so that it will not atrophy. The member is also subjected to flexure from the tibial plate which is also supported on a cut section of the corticalis surrounding the spongiosa. The reinforcing member can be planoconvex or biconvex in cross-section. The reinforcing member can be metal or a resin reinforced with carbon fibre. The reinforcing member is placed between the tibial plate and a conventional plastic plate member having a top surface shaped to engage with the femur or femoral prosthesis.
U.S. Pat. No. 4,822,362 describes a tibial prosthesis comprising a metal base plate contoured and dimensioned to correspond to the size and shape of the tibial plateau. The plate has a fixation pin and other projections which extend into the bone to fix and prevent rotation of the plate in use. A plastic cap is provided on the base plate and is made from high molecular weight polyethylene. The plastic cap has a concave upper surface for receiving the femur or a femoral prosthesis. The other surface of the plastic cap is shaped so as to form a snap-fit into the upper surface of the plate.
A tibial plateau prosthesis is described in U.S. Pat. No. 4,769,040. The prosthesis comprises a base plate which is shaped to fit onto the sectioned tibia. The base plate is flexible in the axial direction but stiff in the transverse direction. The distribution of stresses is said to mimic that of natural bone. A conventional articulating upper surface provided by high density polyethylene is present on the base plate.
U.S. Pat. No. 4,673,407 describes a prosthetic device, especially a tibial prosthesis, which comprises an implant for sealing onto the bone surface and an attachment element adapted to be secured to a portion of the base. The attachment member is connected to the implant by a connection member adapted to allow substantially unhindered movement of the implant relative to the attachment member. A low-modulus spring is operatively interposed between the connecting member and the implant so as to bias the implant against the bone surface. Movement of the implant allows substantially physiological loading of the bone in response to compression, shear, rotational and twisting forces.
U.S. Pat. No. 4,479,271 describes a tibial prosthesis which is adapted to promote the ingrowth of bone and/or tissue into the portion of the prosthesis which is in contact with the bone. The prosthesis comprises a bottom fibrous metal layer, an intermediate reinforcing metal layer and a molded, contoured polyethylene top layer.
U.S. Pat. No. 4,462,120 describes a total knee prosthesis. The tibial portion includes a metal base plate and two discrete pads which can be of different thicknesses and are made from polyethylene.
U.S. Pat. No. 4,268,920 discloses a knee joint endoprosthesis. The tibial and femoral components are arranged with respect to each other to permit a degree of rotation in the joint.
A knee joint endoprosthesis is described in European Patent No. 46926. The prosthesis has a tibial component which comprises two individual parts. One part carries a joint sliding surface, that is the surface which interacts with the femur or femoral prosthesis, and the other part carries an anchoring surface for anchoring the component to the bone. The two parts are connected to each other by the interposition of an elastic damping material for the transmission of compressive thrust and torsional forces. The connection between the two parts consists exclusively of the elastic damping material. The elastic damping material can be in the form of a plate or annular shaped or a plurality of elastic bodies which may be cylindrical, spherical or parallelapipedal shaped. The material may also vary in elasticity in different parts. The elastic material is preferably a silicone rubber.
French Patent No. 2585236 describes a total knee joint prosthesis. The tibial implant comprises a friction resistant plastic plate carried by a metal support. The bottom of the metal support has a series of dove-tail inserts to increase the surface of the support which is sealed to the tibia. The plastic plate has a lower projection which fits into a corresponding cavity in the support plate. The plastic plate when placed on the support is rendered immobile by the interaction of the projection and cavity.
European Patent No. 46926 discloses a knee prosthesis in which in the tibial component there is an elastic damping material. This material is placed between a metal support and a plastic part. The metal support has a surface for anchorage against the tibia. The plastic part has one surface shaped for sliding contact with the femur or a femoral prosthesis. Between the two parts is an elastic damping material which connects them together. The elastic damping material may be in the form of a plate or may be annular shaped.